Flow force compensation

ABSTRACT

In load sensing systems, pressure compensating valves are normally used to maintain a predetermined pressure differential across the orifices in the directional control valve at a predetermined level. The predetermined pressure level acting across the directional control valve can be adversely affected by flow forces acting on a valving element of the pressure compensating valve. In the subject invention, flow force compensation is provided by having a forced balancing device connected to a pressure compensated valve device so that a force that is directly proportional to the degree of flow forces acting on the pressure compensating valve device is transferred to a valving element thereof in opposition to the flow forces that are acting on the valving element. The subject arrangement is also applicable to providing flow force compensation for negative load pressure compensating valve devices. By having flow force compensation that is directly responsive to a predetermined differential pressure across a directional control valve, the predetermined pressure differential thereacross can be more precisely maintained regardless of varying load conditions in the fluid system.

TECHNICAL FIELD

This invention relates generally to a pressure compensated flowcontrolled fluid system, and more particularly to controlling the flowforces acting on pressure compensated valves in the system.

BACKGROUND ART

It is well known in the hydraulic art to have a fluid system wherein thedelivery flow rate of the pump is controlled so as to provide the neededflow to an actuator at a pressure higher by a fixed value than thepressure required to move the load. This type of system is typicallycalled a load sensing system. Likewise, it is well-known to provide apressure compensating valve in each pressure line that supplies fluid torespective directional control valves in order to maintain apredetermined differential pressure across an orifice in the directionalcontrol valve regardless of variations in the load. By maintaining apredetermined differential pressure across the orifice of thedirectional control valve, the volume of fluid passing therethroughremains constant for a given orifice size irrespective of changing loadconditions. This type of pressure compensating valve is normallyreferred to as a positive load pressure compensating valve. The term"positive load" refers to a system wherein the load is a resisting typeof load. It is also well known to use a negative load pressurecompensating valve in the exhaust flow line to control the rate of fluidflow across the directional control valve during conditions in which theload is an aiding type load. In an aiding type load, the load isattempting to exhaust the fluid from the actuator faster than the fluidis being introduced thereto. This operating condition is referred to asa "negative load".

The different types of pressure compensating valves noted above have avalving element therein which serves as a variable restriction in orderto control the fluid flow thereacross to maintain the constantdifferential pressure across the orifice in the directional controlvalve. It is well known that as the pressure drop across the valvingelement increases, a force, normally referred to as a flow force, iscreated which acts on the valving element adversely affecting itsoperation. In most cases involving pressure compensating valves, thisunwanted force acts to prematurely cause the valving element to furtherrestrict the flow thereacross. This unwanted restriction results in thedifferential pressure across the directional control valve beingchanged. Normally this change is in the direction of reducing thedesired differential pressure. U.S. Pat. No. 5,150,574 dated Sep. 29,1992 by Toichi Hirata et al. attempts to overcome this problem byproviding a counteracting force to the pressure compensating valve thatis established by a relationship between the pressures upstream anddownstream thereof. This arrangement appears to at least partiallyoffset the flow forces, but due to the pressure acting oncross-sectional areas that are unequal in size, the resultingcounteracting force is not directly proportional to the varyingdifference in the pressure drop across the pressure compensating valve.Furthermore, this counteracting force is, at least in part, alwaysresponsive to the highest system pressure. Consequently, its affect onany other circuit in the system that is being loaded at a lesser amountwould be receiving a counteracting force to the valving element of thepressure compensating valve that is not typical for that particularcircuit.

The present invention is directed to overcoming one or more of theproblems as set forth above.

DISCLOSURE OF THE INVENTION

In one aspect of the present invention, a flow force compensating meansis provided for use in a fluid system having a source of pressurizedfluid connected to a reservoir, an actuator driven by fluid from thesource of pressurized fluid, a directional control valve having variableorifice means disposed therein, and pressure compensating valve meanshaving valving element means with throttling orifices and beingoperative to control a differential pressure across the variable orificemeans of the directional control valve at a substantially constant rate.The flow force compensating means includes force balancing meansresponsive to the differential pressure across the variable orificemeans of the directional control valve for generating and transmitting abalancing force to the valving element of the pressure compensatingvalve means to offset the flow forces acting on the valving element ofthe pressure compensating valve means.

The present invention provides a flow force compensating means thatprovides a balancing force to the compensating valve means that isdirectly related to the desired differential pressure established acrossthe directional control valve. Consequently, by providing a balancingforce that is directly related to the differential pressure across thedirectional control valve, the flow forces that are generated as aneffect of the flow across the pressure compensating valve aresubstantially eliminated. Therefore, the desired pressure differentialacross the directional control valve is maintained, thus allowing, ifdesired, the opportunity to maintain a lower pressure differentialacross the main control valve.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial diagrammatic and partial schematic representation ofa fluid system incorporating an embodiment of the present invention; and

FIG. 2 is a partial diagrammatic and partial schematic representation ofanother embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to the drawings and more specifically to FIG. 1, a fluidsystem 10 is shown and includes, a source of pressurized fluid 12, suchas a flow pressure compensated pump 13 connected to a reservoir 14, asystem relief valve 15 and first and second load sensing circuits 16,18.The first load sensing circuit 16 is connected to the source ofpressurized fluid 12, by two conduits 20,22 while the second loadsensing circuit 18 is connected to the source of pressurized fluid 12 bythe conduit 20 and a conduit 24. The first load sensing circuit 16includes a pressure compensating valve means 26 having a force balancingmeans 28 connected thereto, a directional control valve 30, and anactuator 32.

The second load sensing circuit 18 includes a pressure compensatingvalve means 36 having a force balancing means 38 connected thereto, adirectional control valve 40, and an actuator 42. The first and secondload sensing circuits 16,18 are substantially the same and the variouselements therein are substantially the same.

The pressure compensating valve means 26 includes a positive loadpressure compensating valve 46 having a valving element means 48,slidably disposed therein to define pressure chambers 49,50 at oppositeends thereof. A biasing member 50 is disposed in one pressure chamber 49of the pressure chambers 49,50 and operable to bias the valving elementmeans 48 in one direction. The positive load pressure compensator valve46 has an inlet port 52 and an outlet port 54, each in operativecommunication with the valving element means 48. The valving elementmeans 48 has a throttling orifice 56 disposed thereon and operative inuse to control fluid flow thereacross in response to movement of thevalving element means 48. A passageway 58 is defined in the valvingelement means 48 and operative to communicate the pressurized fluid atthe outlet port 54 to the other pressure chamber 50 of the pressurechambers 49,50.

The conduit 22 from the source of pressurized fluid 12 is connected tothe inlet port 52 of the positive load pressure compensated valve 46while a pressure supply conduit 62 connects the outlet port 54 thereofwith the directional control valve 30. A pair of conduits 64,66interconnect the actuator 32 and the directional control valve 30 whilean exhaust conduit 68 connects the directional control valve 30 with thereservoir 14.

The directional control valve 30 has variable orifice means 70 definedtherein operative to control fluid flow thereacross. The variableorifice means 70 includes adjustable throttling orifices 71A,71B. A loadpressure sensing passageway 72 is defined therein and is in selectivecommunication with the pressure of the fluid in the actuator 32 and thereservoir 14. The load sensing passageway 72 is connected to the flowpath at a location downstream of the respective adjustable throttlingorifices 71A,71B. The directional control valve 30 is movable between aneutral position and first and second operative positions. In theneutral position, the pressure supply conduit 62 is blocked fromcommunication with the conduits 64,66 and the load pressure sensingpassageway 72 is connected to the reservoir 14 through the exhaustconduit 68. In the first operative position, the pressure supply conduit62 is communicated with the conduit 64 by way of the adjustablethrottling orifices 71A and the load pressure sensing passageway 72 iscommunicated therewith downstream of the adjustable throttling orifice71A while the conduit 66 is connected with the exhaust conduit 68. Inthe second operative position, the pressure supply conduit 62 iscommunicated with the conduit 66 across the adjustable throttlingorifice 71B and the load pressure sensing passageway 72 is communicatedwith the flow path downstream of the adjustable throttling orifice 71Bwhile the conduit 64 is connected with the exhaust conduit 68. A signalconduit 74 interconnects the load pressure sensing passageway 72 withthe one pressure chamber 49 of the positive load pressure compensatingvalve 46.

The force balancing means 28 which is connected to the pressurecompensating valve means 26 can be an integral part thereof or aseparate member connected thereto. In the subject illustration, theforce balancing means 28 is an integral part of the positive loadpressure compensating valve 46 in the form of a balancing pistonassembly 76. The balancing piston assembly 76 includes a piston 78slidably disposed therein. A pair of pressure responsive chambers 80,82are defined therein at opposite ends of the piston 78. A biasing member84 is disposed in one pressure responsive chamber 80 of the pressureresponsive chambers 80,82 and operative to bias the piston 78 in onedirection. A force transfer slug 86 is slidably disposed between thebalancing piston assembly 76 and the positive load pressure compensatingvalve 46. The force transfer slug 86 is in operative contact with theend of the piston 78 in the other pressure responsive chamber 82 of thebalancing piston assembly 76 and the end of the valving element 48 inthe one pressure chamber 49 of the positive load pressure compensatingvalve 46. A signal conduit 90 connects the load pressure in the signalconduit 74 with the one pressure responsive chamber 80 while a signalconduit 92 connects the pressurized fluid in the pressure supply conduit62 with the other pressure responsive chamber 82. The signal conduit 92is connected to the pressure supply conduit 62 at a location immediatelyadjacent the variable orifice means 70 of the directional control valve30. A signal conduit 94, a signal resolver 96, and a signal conduit 98connects the load pressure signal in the load pressure sensingpassageway 72 by way of conduit 74 with a flow-pressure compensatedcontrol 100 of the flow pressure compensated pump 13.

The pressure compensating valve means 36 and the force balancing means38 of the second load sensing circuit 18 are identical to the pressurecompensating valve means 26 and the force balancing means 28 of thefirst load sensing circuit 16. Therefore, all elements in the secondload sensing circuit 18 that corresponds with those of the first loadsensing circuit 16 have like element numbers. Likewise, the directionalcontrol valve 40 and the actuator 42 of the second load sensing circuit18 are identical to the directional control valve 30 and the 10 actuator32 of the first load sensing circuit 16. In the second load sensingcircuit 18, the load signal conduit 74 is connected with the resolver 96by a signal conduit 102 and the conduit 90. All other conduits andconnections are the same as described with respect to the first loadsensing circuit 16.

Referring now to FIG. 2, another embodiment of the present invention isillustrated. The fluid system 10 of FIG. 2 has many similarities to thefluid system of FIG. 1. Like FIG. 1, the fluid system 10 of FIG. 2includes the source 12 of pressurized fluid, the reservoir 14, thesystem relief valve 15, and the first and second load sensing circuits16,18. Also like FIG. 1, the fluid circuits 16,18 each contain therespective pressure compensating valve means 26,36, the force balancingmeans 28,38, the directional control valves 30,40 having the variableorifice means 70 defined therein with the adjustable throttling orifices71A,71B, and the actuator 32,42. The respective pressure compensatingvalve means 26,36 and the force balancing means 28,38 of each of theload sensing circuits 16,18 have the positive load pressure compensatingvalve 46 and the balancing piston assembly 76 identical to thatdisclosed with respect to FIG. 1. The positive load pressurecompensating valve 46 and the balancing piston assembly 76 are connectedin the respective circuits 16,18 identical to that disclosed withrespect to FIG. 1.

The pressure compensating valve means 26 includes a negative loadpressure compensating valve 106 operatively connected in the exhaustconduit 68 of the first loads sensing circuit 16. Likewise, the loadsensing circuit 18 includes a negative load pressure compensating valve108 operatively connected in the exhaust line 68 thereof. Each of therespective negative load pressure compensating valves 106,108 has thebalancing piston assemblies 76 connected thereto. The balancing pistonassemblies 76 of FIG. 2 are identical to those of FIG. 1.

The negative load pressure compensating valve 106 has a valving elementmeans 110 slidably disposed therein and has throttling orifice 112thereon for controlling fluid flow through the exhaust conduit 68 inresponse to movement of the valving element means 110. A pair ofpressure chambers 114,116 are defined in the negative load pressurecompensated valve 106 at opposite ends of the valving element means 110.A biasing member 118 is disposed in one pressure chamber 114 of thepressure chambers 114,116 and operative to bias the valving elementmeans 110 in one direction.

The variable orifice means 70 of the directional control valve 30further includes adjustable exhaust throttling orifices 119A,119Bdisposed therein and operative to control the exhaust fluid flow fromthe actuator 32 to the exhaust conduit 68 in a well known manner. Anegative load pressure sensing passageway 120 is operatively connectedto the exhaust flow line upstream of the respective adjustable exhaustthrottling orifices 119A,119B. A signal conduit 122 connects thenegative load pressure sensing passageway 120 with the other pressurechamber 116 of the negative load pressure compensating valve 106. Asignal conduit 123 connects the signal conduit 122 with the otherpressure responsive chamber 82 of the balancing piston assembly 76. Theexhaust conduit 68 is connected from the directional control valve 30 toan inlet port 124 of the negative load compensating valve 106 while aportion of the exhaust conduit 68 is connected between an outlet port126 of the negative load pressure compensating valve 106 and thereservoir 14. A signal conduit 128 connects the exhaust conduit 68upstream of the respective adjustable exhaust throttling orifices119A,119B to the one pressure chamber 114 of the negative load pressurecompensating valve 106. A signal conduit 130 connects the conduit 68upstream of the respective adjustable exhaust throttling orifices119A,119B to the one pressure responsive chamber 80 of the balancingpiston assembly 76.

The negative load pressure compensating valve 108 and the balancingpiston assembly 38 of the second load sensing circuit 18 are identicalto the corresponding ones in the first load sensing circuit 16.Likewise, the directional control valve 40 and the actuator 42 of thesecond load sensing circuit 18 are identical to that of the first loadsensing circuit 16.

It is recognized that various forms of the fluid system 10 could beutilized without departing from the essence of the invention. Forexample, the cross-sectional area of the piston 78 of the balancingpiston assembly 76 could be different from the cross-sectional area ofthe valving element means 48 of the pressure compensating valve means46. Likewise, the biasing force of the biasing member 84 of thebalancing piston assembly 76 could be different from the spring force ofthe biasing member 51 of the pressure compensating valve means 26. It isalso recognized that various combinations of cross-sectional areas andspring forces could be utilized without departing from the essence ofthe invention. Furthermore, it is recognized that the signal conduit 123which connects the signal from the signal conduit 122 to the end of thepiston 78 in the other pressure responsive chamber 82 of the balancingpiston assembly 76 could be connected immediately adjacent theadjustable exhaust throttling orifices 119 in order to improve systemresponse and to make the forced balancing means 28 respond moreprecisely to the differential pressure across the adjustable throttlingorifices 119A,119B.

Industrial Applicability

In the operation of the fluid system 10, the flow-pressure compensatedpump 13 delivers pressurized fluid from the reservoir 14 through theconduits 20,22,24, the respective pressure compensating valve means26,36, and the conduits 62 to the respective directional control valve30,40. With both of the directional control valves 30,40 in theirneutral, flow blocking position, the flow-pressure compensated control100 conditions the flow-pressure compensated pump 13 to reduce fluidflow output to a level sufficient to replace any fluid leakage and tomaintain the standby pressure at a predetermined minimum pressure level.

With the directional control valves 30,40 in their neutral, flowblocking positions, each of the valving element means 48 of the positiveload pressure compensating valves 46 moves to a flow blocking positionat which communication from the inlet port 52 to the outlet port 54 issubstantially blocked. The valving element means 48 achieves a balancedposition at which a predetermined pressure level is maintained in thepressure supply conduit 62, as is well known in the art. The valvingelement means 48 moves to its flow blocking position in response to thepressurized fluid at the outlet port 54 being directed through thepassageway 58 of the valving element means 48 to the 10 other pressurechamber 50. The pressurized fluid in the other pressure chamber 50 actson the end of the valving element 48 moving it against the bias of thebiasing member 51. Simultaneously therewith, the pressurized fluid inthe pressure supply conduit 62 immediately adjacent the adjustablethrottling orifice 71A is communicated through the signal conduit 92 tothe other pressure responsive chamber 82 of the balancing pistonassembly 76 and acts on the end of the piston 78 therein. Thepressurized fluid acting on the end of the piston 78 moves it againstthe bias of the biasing member 84. Since the load pressure sensingpassageway 72 is in communication with the reservoir 14 there is nopressure signal available in the signal conduits 74,90. Consequently,there is no pressure available in the one pressure chamber 49 of thepositive load pressure compensating valve 46 to act against the end ofthe valving element means 48 or in the one pressure responsive chamber80 to act against the end of the piston 78.

Since the second load sensing circuit 18 is substantially the same asthat set forth with respect to the first load sensing circuit 16, thefollowing operational description is directed only to the first loadsensing circuit 16.

Upon movement of the directional control valve to its first operativeposition, fluid flow is directed across the adjustable throttlingorifice 71A from the pressure supply conduit 62 to the conduit 64 andsubsequently to the actuator 32 causing it to move. Exhaust flow fromthe actuator 32 is directed to the reservoir 14 through the conduit 66,the directional control valve 30 and the exhaust conduit 68. The flowfrom the pressure supply conduit 62 to the actuator 32 causes thepressure in the conduit to reduce. As a result of the reduced pressurein the pressure supply conduit 62, the pressure in the other pressurechamber 50 of the positive load pressure compensating valve 46 reduces.The valving element means 48 of the positive load pressure compensatingvalve 46 moves towards the position in which fluid flow from the inletport 52 is directed across the throttling orifice 56 thereof to theoutlet port 54. As the fluid flow is being directed from the flowpressure compensated pump 13 to the actuator 32, a signal representativeof the load is directed therefrom through the load pressure sensingpassageway 72, the conduit 94, the resolver 96, and the conduit 98 tothe flow-pressure compensated control 100. The flow-pressure compensatedcontrol 100 operates in a known manner to control the rate of fluid flowand the pressure level of the fluid flow across the adjustablethrottling orifices 71 to the actuator 32. The flow-pressure compensatedcontrol 100 functions to maintain a predetermined differential pressurebetween the flow-pressure compensated pump 13 and the actuator 32. As iswell known, the constant differential pressure across the valves in thefirst load sensing circuit 16 maintains a predetermined flow ratethereacross and the pressure level of the flow-pressure compensated pump13 is operating at a level as determined by the load and at apredetermined level higher than the load.

At the same time, the positive load pressure compensating valve 46functions to maintain a differential pressure across the adjustablethrottling orifice 71A at a predetermined level. The predetermineddifferential pressure level being maintained across the adjustablethrottling orifice 71A of the directional control valve 30 is normallylower than the differential pressure being maintained from theflow-pressure compensated pump 13 to the actuator 32 across the firstload sensing circuit 16. Therefore, the throttling orifice 56 of thevalving element means 48 is throttling the fluid flow from the inletport 52 to the outlet port 54 thereof and maintaining generally apressure drop thereacross equal to the difference in the differentialpressure that the flow pressure compensated control 100 is maintainingfrom the flow-pressure compensated pump 13 to the actuator 32 and thedifferential pressure that the positive load compensating valve 46 ismaintaining across the adjustable throttling orifice 71A.

Since the pressure drop that is being developed across the throttlingorifice 56 is small, any flow forces developed therein trying to furtherrestrict the fluid flow is minimal. However, to the extent that there isflow forces acting on the valving element means 48, they are beingoffset by the flow force balancing means 28. This is accomplished by thepressure representative of the load in the actuator 32 beingcommunicated to the end of the piston 78 in the one pressure responsivechamber 80 acting in combination with the biasing member 84 to bias thepiston 78 against the opposing force created by the pressurized fluidfrom the pressure supply conduit 62 acting on the other end thereof inthe other pressure responsive chamber 82. If the positive load pressurecompensating valve 46 is controlling the differential pressure acrossthe adjustable throttling orifice 71A at a level lower than thepredetermined level as a result of flow forces, the pressurized fluid inthe pressure supply conduit 62 would be below the desired amount.Consequently, the force acting on the end of the piston 78 in the otherpressure responsive chamber 82 would be likewise smaller, thus,upsetting the balance of the forces acting on the piston 78. The higherforce acting on the piston 78 by the combined force of the biasingmember 84 and the load pressure acting on the end of the piston 78 inthe one pressure responsive chamber 80 moves the piston 78 in adirection against the force transfer slug 86. A force equal to theunbalanced force is directly applied through the load transfer slug 86to the end of the valving element means 48. This force acts to move thevalving element means 48 to a position at which the throttling orifice56 are larger. As the valving element means 48 moves to a position atwhich the throttling orifice 56 are larger as a result of the forceacting thereon from the force transfer slug 86, the pressure level inthe conduit 62 is increased. This increased pressure is simultaneouslytransferred to the other pressure responsive chamber 82 to againestablish a balanced condition in the force balancing means 28. Theforce tending to close the throttling orifice 56 of the valving elementmeans 48 is thus offset and the predetermined differential pressureacross the adjustable throttling orifice 71A is maintained at itspredetermined level.

If the second load sensing circuit 18 is operated at the same time asthe first load sensing circuit 16, the difference in pressure across therespective positive load pressure compensating valves 46 can varydrastically. For example, assume that it is desirable to have the samepredetermined differential pressure across the adjustable throttlingorifices 71 of each of the directional control valves 30,40, but thatthe actuator 32 of the first load sensing circuit 16 has a small loadand the actuator 42 of the second load sensing circuit 18 has a loadthat requires twice the amount of force to move. In this situation, theload signal from the second load sensing circuit 18 to the flow-pressurecompensated control 100 functions to condition the flow-pressurecompensated pump 13 to provide the necessary flow at the desiredpressure level to move the heavily loaded actuator 42 at thepredetermined rate of movement by maintaining the differential pressurefrom the flow-pressure compensated pump 13 to the actuator 42 across thesecond load sensing circuit 18. At the same time, as previouslydiscussed with respect to the first load sensing circuit 16, thepositive load pressure compensated valve 46 of the second load sensingcircuit 18 functions to throttle the fluid flow thereacross to maintainthe pressure drop thereacross equivalent to the difference in thedifferential pressure being maintained between the flow-pressurecompensated pump 13 and the load pressure in the actuator 42 and that ofthe differential pressure being maintained across the adjustablethrottling orifice 71A as controlled by the positive load pressurecompensated valve 36.

Since the actuator 32 of the first load sensing circuit 16 is notheavily loaded, its load signal in the signal conduit 94 has no affecton the flow-pressure compensated control 100 since it is lower than thepressure signal in the conduit 102. However, the load pressure due tothe load on the actuator 32 is still being directed through the loadpressure sensing passageway 72 of the directional control valve 30 tothe one pressure chamber 49 of the positive load pressure compensatedvalve 46 and also through the signal conduit 90 to the one pressureresponsive chamber 80 of the force balancing means 28. Furthermore,since the actuator 32 is lightly loaded, the flow from the flow-pressurecompensated pump 13 tends to flow to the actuator 32 having the lightestload. However, in this instance the added flow trying to cross theadjustable throttling orifice 71A of the first load sensing circuit 16results in an increase in pressure in the pressure supply conduit 62.This increase in fluid pressure in the pressure supply conduit 62 isdirected through the passageway 58 within the valving element means 48to the other pressure chamber 50 therein thus acting on the end of thevalving element means 48 tending to move it towards the flow blockingposition. The movement of the valving element means 48 is opposed by thecollective forces of the biasing member 51 and the pressurized fluid inthe one pressure chamber 49 acting on the other end thereof. The valvingelement means 48 achieves a balanced position at which the differentialpressure acting across the adjustable throttling orifice 71A ismaintained at the predetermined pressure level.

In this particular situation, the pressure level of the fluid in thepressure supply conduit 62 of the first load sensing circuit 16 issubstantially less than the pressure level in the conduit 22 upstream ofthe positive load pressure compensating means 46. Since the load on theactuator 32 is half of the load on the actuator 42, the pressure levelin the pressure supply conduit 62 of the first load sensing circuit 16downstream of the positive load pressure compensating valve 46 isapproximately half of the pressure level of the fluid in the conduit 22upstream of the positive load pressure compensating control valve 46.Therefore, the pressure drop across the throttling orifice 56 thereof ishigh. Since, as is well known, the flow forces acting on a valve isdirectly proportional to the pressure drop thereacross, the forcerepresentative of the flow forces acting to close the throttling orifice56 of the valving element means 48 is high. If the force trying to closethe throttling orifice 56 is not counteracted, the differential pressureacross the adjustable throttling orifice 71A will be operating at alevel lower than that desired. The force balancing means 28 functions tosense the lowering of the differential pressure across the directionalcontrol valve 30 and provide a counteracting force to the valvingelement means 48 returning it to the position at which the predetermineddifferential pressure across the adjustable throttling orifice 71A isonce again maintained.

Since any forces resulting from flow forces acting to close the valvingelement means 48 results in the pressure level of the fluid in pressuresupply conduit 62 being lowered, the lower pressure level therein issensed by the force balancing means 28 and results in the balancingpiston assembly 76 thereof becoming unbalanced. Since the balancingpiston assembly 76 operates in a balanced mode only when thepredetermined differential pressure across the adjustable throttlingorifice 71A is maintained, the force equivalent to the degree ofunbalance is transmitted through the force transfer slug 86 to thevalving element means 48 in opposition to the force created by flowforces tending to close the valving element means 48. As the valvingelement means 48 moves to increase the opening of the throttling orifice56, the pressure level in the pressure supply conduit 62 once againincreases and the increase in pressure in the pressure supply conduit 62is directed through the conduit 92 to act on the end of the piston 78 inthe other pressure responsive chamber 82 to once again place the piston78 in its balanced condition. This balanced condition, as noted above,is representative of the differential pressure acting across theadjustable throttling orifice 71A. Therefore, in the subjectarrangement, any flow forces attempting to reduce the fluid flow acrossthe throttling orifice 56 is quickly sensed and offset by the balancingpiston assembly 76.

By having the signal conduit 92 connected to the pressure in thepressure supply conduit 62 immediately adjacent the variable orificemeans 70, the effects of the frictional losses in the pressure supplyconduit 62 can be offset thus improving the response time of therespective load sensing circuits 16,18. The response is likewiseimproved by increasing the velocity of the fluid flow being deliveredfrom the pressure compensating means 26 to the directional control valve30. This may be accomplished by reducing the size of the pressure supplyconduit 62 therebetween. The objective is to decrease the volume offluid in the pressure supply conduit 62 while increasing the velocity ofthe fluid. This volume of fluid is one of the most critical parameterseffecting system response time while not affecting the predetermineddifferential pressure across the variable orifice means 70.

Referring to FIG. 2, the operation of this fluid system 10 is identicalto the operation of the fluid system 10 of FIG. 1 when the system ishandling the positive load. In the event, that the fluid system 10 ofFIG. 2 is handling a negative load, the negative load pressurecompensating valves 106,108 function to control the fluid flow acrossthe respective adjustable exhaust throttling orifices 119A,119B of therespective directional control valves 30,40 at a predetermineddifferential pressure level.

The operation of the negative load pressure compensating valve 106,108is well known. In general, the return flow from the respective actuators32,42 is directed across the respective adjustable exhaust throttlingorifices 119A,119B, through the exhaust conduit 68, and through thenegative load pressure compensating valve 106,108 to the reservoir 14.The valving element means 110 of the respective negative load pressurecompensating valves 106,108 has the throttling orifice 112 to controlthe fluid flow thereacross in response to movement of the valvingelement means 110. The valving element means 110 moves to a fluid flowblocking position against the bias of the biasing member 118 in responseto a negative load pressure signal received through the signal conduit122 and the negative load pressure sensing passageway 120 that isconnected to the exhaust flow upstream of the adjustable exhaustthrottling orifices 119A,119B. The pressure downstream of the adjustableexhaust throttling orifices 119A,119B is connected through the signalconduit 128 to the one pressure chamber 114 and acts in cooperation withthe biasing member 118 to control the fluid flow across the throttlingorifice 112 to maintain the predetermined differential pressure acrossthe respective adjustable exhaust throttling orifices 119A,119B.

As previously noted with respect to the balancing piston assembly 76connected to the positive load pressure compensator 46 of FIG. 1, thebalancing piston assembly 76 is likewise connected to the negative loadpressure compensating valve 106 and functions in the same manner tocounteract any flow forces that tends to close the valving element means110. The piston 78 of the subject balancing piston assembly 76 ismaintained in a balanced condition by the pressurized fluid upstream ofthe respective adjustable exhaust throttling orifices 119A,119B beingcommunicated to the other pressure responsive chamber 82 to act on theend of the piston 78 while the pressurized fluid downstream of therespective adjustable exhaust throttling orifices 119A,119B iscommunicated through the signal conduit 130 to the one pressureresponsive surface 80 to act on the other end of the piston 78 incooperation with the biasing member 84. The piston 78 is maintained andbalanced as long as the predetermined differential pressure across theappropriate one of the adjustable exhaust throttling orifices 119A,119Bis maintained at the predetermined differential pressure level. If thepressure level in the exhaust conduit 68 upstream of the throttlingorifice 112 of the valving element means 110 increases due to flowforces tending to close the throttling orifice 112, the resultingincreased pressure is directed against the end of the piston 78 in theone pressure responsive chamber 80 causing an unbalance in forces actingon the piston 78. The magnitude of the unbalanced force is transferredthrough the force transfer slug 86 to one end of the valving elementmeans 110 to counteract the flow forces tending to close the throttlingorifice 112. As previously noted with respect to FIG. 1, the magnitudeof force created by flow across the throttling orifice 112 increases asthe pressure drop across the throttling orifice 112 increases.Consequently, the higher the flow forces tending to close the valvingelement means 110, the greater the increase in pressure in the conduit68 and likewise in the one pressure responsive chamber 80. The force ofthe fluid pressure acting on the end of the piston 78 is transferredthrough the load transfer slug 86 to the end of the valving elementmeans 110 to offset the forces caused by the flow forces acting to closethe throttling orifice 112.

In view of the foregoing, it is readily apparent that the fluid system10 of the present invention provides flow force compensation to thepressure compensating valve means 26,36 that is directly proportional tothe differential pressure acting across the variable orifice means 70 ofthe directional control valves 30,40. This compensation of the flowforces maintains the predetermined differential pressure across thedirectional control valves 30,40 regardless of major differences in themagnitude of the loads between the various circuits in the system.

Other aspects, objects and advantages of the invention can be obtainedfrom a study of the drawings, the disclosure and the appended claims.

I claim:
 1. Flow force compensating means for use in a fluid systemhaving a source of pressurized fluid connected to a reservoir, anactuator driven by fluid flow from the source of pressurized fluid, adirectional control valve having variable orifice means disposedtherein, and pressure compensating valve means having valving elementmeans with a throttling orifice defined thereon and being operative tocontrol a differential pressure across the variable orifice means of thedirectional control valve at a substantially constant level bycontrolling the flow across the pressure compensating valve means, theflow force compensating means comprising:force balancing meansresponsive only to the differential pressure across the variable orificemeans of the directional control valve for generating and transmitting abalancing force to the valving element means of the pressurecompensating valve means to offset the flow forces acting on the valvingelement means thereof.
 2. Flow force compensating means for use in afluid system having a source of pressurized fluid connected to areservoir, an actuator driven by fluid flow from the source ofpressurized fluid, a directional control valve having variable orificemeans disposed therein, and pressure compensating valve means havingvalving element means with a throttling orifice defined thereon andbeing operative to control a differential pressure across the variableorifice means of the directional control valve at a substantiallyconstant level by controlling the flow across the pressure compensatingvalve means, the flow force compensating means comprising:forcebalancing means responsive only to the differential pressure across thevariable orifice means of the directional control valve for generatingand transmitting a balancing force to the valving element means of thepressure compensating valve means to offset the flow forces acting onthe valving element means thereof, the force balancing means isoperatively connected to the pressure supply conduit and includes abalancing piston assembly having a piston slidably disposed therein todefine pressure responsive chambers at opposite ends thereof, a biasingmember in contact with the end of the piston in the one pressureresponsive chamber, and a force transfer slug disposed between the endof the piston in the other pressure responsive chamber and the valvingelement means of the pressure compensating valve means.
 3. The flowforce compensating means of claim 2 wherein the end of the piston in theone pressure responsive chamber is operatively connected with thepressurized fluid from the source of pressurized fluid at a locationdownstream of the variable orifice means of the directional controlvalve and the end of the piston in the other pressure responsive chamberis operatively connected to the pressurized fluid from the source ofpressurized fluid upstream of the variable orifice means of thedirectional control valve.
 4. The flow force compensating means of claim3 wherein the valving element means of the pressure compensated valvemeans has a predetermined effective cross sectional area and the pistonof the force balancing means has a predetermined effectivecross-sectional area that is substantially equal to the cross sectionalarea of the valving element means of the pressure compensated valvemeans.
 5. The flow force compensating means of claim 4 wherein thepressure compensated valve means has a biasing member with apredetermined effective force in contact with the valving element meanstherein and the biasing member of the force balancing means has apredetermined effective force substantially equal to the force of thebiasing member in the pressure compensated valve means.
 6. The flowforce compensating means of claim 5 wherein the variable orifice meansin the directional control valve includes an adjustable throttlingorifice located between the source of pressurized fluid and theactuator, the pressure compensated valve means includes a positive loadpressure compensated valve having the balancing piston assemblyconnected thereto and which is located between the source of pressurizedfluid and the directional control valve, a pressure supply conduitconnects the pressure compensating valve means with the directionalcontrol valve and the end of the piston in the other pressure responsivechamber is operatively connected to the pressure supply conduit at alocation immediately adjacent the adjustable throttling orifice of thedirectional control valve.
 7. The flow force compensating means of claim5 wherein the variable orifice means in the directional control valveincludes an adjustable exhaust throttling orifice located between theactuator and the reservoir, and the pressure compensated valve meansincludes a negative load pressure compensated valve having anotherbalancing piston assembly connected thereto and which is located betweenthe adjustable exhaust throttling orifice and the reservoir.
 8. The flowforce compensating means of claim 5 wherein the variable orifice meansof the directional control valve includes an adjustable throttlingorifice located between the source of pressurized fluid and the actuatorand an adjustable exhaust throttling orifice located between theactuator and the reservoir, the pressure compensating valve means withthe force balancing means connected thereto includes a positive loadpressure compensating valve with the balancing piston assembly connectedthereto is located between the source of pressurized fluid and thedirectional control valve and a negative load pressure compensated valvemeans with the balancing piston assembly connected thereto is locatedbetween the directional control valve and the reservoir.